With modern plumbing and hygiene, the number of nasty microbes we humans are exposed to has plummeted, while the rate of autoimmune diseases and allergies has shot up. Are those related? Proponents of the hygienehypothesis think so: our immune system is supposed to develop by encountering microbes, so being too clean throws it out of whack as the immune system overreacts to minor insults.

A new study found that mice raised germ-free had especially high numbers of invariant natural killer T cells (iNKT) in their colons and lungs—the mouse versions of inflammatory bowel disease and asthma, respectively. Most evidence supporting the hygiene hypothesis has just been in observed correlations, so this research that identifies a plausible molecular mechanism is good evidence for how over-cleanliness might cause immune dysfunction.

The researchers homed in on a signaling molecule called CXCL16, which is associated with the build-up of iNKT cells. iNKT cells identify what cells that seem to be pathogens—what might be harmless cells when it comes to inflammatory bowel disease or asthma—and release a flood of chemicals that cause inflammation. The researchers determined that CXCL16 is highly expressed in germ-free mice, and they suggest that normal levels of microbial exposure suppresses levels of CXCL16 and thus also iNKT cells.

Interestingly, microbial exposure only affected immune system development during a critical period in childhood. Microbial exposure had no effect on the iNKT cells of adult germ-free mice, but it did restore iNKT levels to normal in the offspring of pregnant germ-free mice exposed just before birth.

While this study does add a compelling explanation for how the hygiene hypothesis may play out in our bodies, it’s worth noting that human children living outside of bubbles are never as germ free as mice in a sterile lab. So it’s not yet clear that this particular effect happens to real children in the real world.

So what’s new? My father, a famed medical researcher, preached this in the 1930’s, but no one was listening.

http://n/a Ron Franklin

I’ve espoused this idea for years. I’ve had lots of people ( including Medics) ask how I stay so healthy.

I always give the same answer, “I eat a little dirt.” Which simply means that I don’t remove every little sliver in my fingers until they have festered a little. That way, my system can build up a little more in the ” immune me part”. & don’t get the raw veggies “to” clean. IMHO.

I wise old commercial fishing Captain passed this idea to me 50 some years ago.

It seems to work, for me.

Annie Marie

Discover proudly presents another series of: Previously on Discover. Sorry guys, that was mean. But in all seriousness: It IS fun to read this one again! It´s a great article! So thank you guys.

Amos Zeeberg (Discover Web Editor)

Well, yes, the hygiene hypothesis isn’t exactly new. But it’s a new study! That “how” in the headline is the new bit: we’re apparently getting close to an explanation.

Hugh Miller

I remember the story my Mother told about my older brother getting sick often! When she went to her local family doctor, this was late 30’s early 40’s, he told her “Elizabeth, you are keeping this child too clean, you got to let him get dirty so he will develop reisitance to normal germs.” Subsequently I, the second child, rarely got sick and those immunities seems to have held into my 60’s.

Sunny D

How are allergies caused by a weak immune system? Is it because the body is overreacting to the tiniest stimulus that it thinks is a microbe? Or just because there are more killer T cells?